If you’re wanting to allow a certain user to make edits to page, you can set their user role to Contributor to allow them to edit their own posts. If you also want them to be able to publish, set their role to Author.
]]>
There are a number of both free and commercial plugins that should be able to accomplish this, which you can find by searching.
However restricting editing to just one paragraph, strictly speaking, is probably not possible.
Hope that helps!
]]>